In situ observation of competitive growth of α grains during β → α transformation in laser beam manufactured TiAl alloys


Low ductility has long been the bottleneck for high temperature application of TiAl alloys. It is reported that grain refinement through boride could improve its mechanical property. However, this refinement is suppressed at high cooling rate. This article delineates for the first time an in situ observation by synchrotron X-ray diffraction. It illustrates the mechanism of competitive nucleation and grain growth of Burgers and non-Burgers α grains during β → α transformation in a Ti–45Al–5Nb–0.2C–0.2B alloy (TNB-V5). Comparing with those in base material, the volume fraction and size of borides are significantly reduced in the welding zone. The non-Burgers α grains nucleate earlier than Burgers α. However, Burgers α grains grow much faster than non-Burgers α during β → α transformation in the welding zone, due to a high thermodynamic driving force of Burgers α grains. No orientation relationships between α and borides or between β and borides are observed in the fast cooling.
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